Add tests to demonstrate nil error in content type matcher failure message

Note: this is most likely not the way we want to test our matchers, but it
demonstrates the current issue.

spec/paperclip/matchers/validate_attachment_content_type_matcher_spec.rb

@@ -17,22 +17,26 @@ describe Paperclip::Shoulda::Matchers::ValidateAttachmentContentTypeMatcher do
 
   it "rejects a class with no validation" do
     expect(matcher).to_not accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it 'rejects a class when the validation fails' do
     Dummy.validates_attachment_content_type :avatar, content_type: %r{audio/.*}
     expect(matcher).to_not accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it "accepts a class with a matching validation" do
     Dummy.validates_attachment_content_type :avatar, content_type: %r{image/.*}
     expect(matcher).to accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it "accepts a class with other validations but matching types" do
     Dummy.validates_presence_of :title
     Dummy.validates_attachment_content_type :avatar, content_type: %r{image/.*}
     expect(matcher).to accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it "accepts a class that matches and a matcher that only specifies 'allowing'" do
@@ -40,6 +44,7 @@ describe Paperclip::Shoulda::Matchers::ValidateAttachmentContentTypeMatcher do
     matcher = plain_matcher.allowing(%w(image/png image/jpeg))
 
     expect(matcher).to accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it "rejects a class that does not match and a matcher that only specifies 'allowing'" do
@@ -47,6 +52,7 @@ describe Paperclip::Shoulda::Matchers::ValidateAttachmentContentTypeMatcher do
     matcher = plain_matcher.allowing(%w(image/png image/jpeg))
 
     expect(matcher).to_not accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it "accepts a class that matches and a matcher that only specifies 'rejecting'" do
@@ -54,6 +60,7 @@ describe Paperclip::Shoulda::Matchers::ValidateAttachmentContentTypeMatcher do
     matcher = plain_matcher.rejecting(%w(audio/mp3 application/octet-stream))
 
     expect(matcher).to accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   it "rejects a class that does not match and a matcher that only specifies 'rejecting'" do
@@ -61,6 +68,7 @@ describe Paperclip::Shoulda::Matchers::ValidateAttachmentContentTypeMatcher do
     matcher = plain_matcher.rejecting(%w(audio/mp3 application/octet-stream))
 
     expect(matcher).to_not accept(Dummy)
+    expect { matcher.failure_message }.to_not raise_error
   end
 
   context "using an :if to control the validation" do
@@ -75,12 +83,14 @@ describe Paperclip::Shoulda::Matchers::ValidateAttachmentContentTypeMatcher do
       dummy = Dummy.new
       dummy.go = true
       expect(matcher).to accept(dummy)
+      expect { matcher.failure_message }.to_not raise_error
     end
 
     it "does not run the validation if the control is false" do
       dummy = Dummy.new
       dummy.go = false
       expect(matcher).to_not accept(dummy)
+      expect { matcher.failure_message }.to_not raise_error
     end
   end