Rely on URI content_type only when provided.

As described in #2118, `OpenURI::Meta#content_type` will return
`application/octet-stream` when no `content-type` header is set. Using
the original `meta` fixes this issue.

lib/paperclip/io_adapters/uri_adapter.rb

@@ -28,9 +28,7 @@ module Paperclip
     end
 
     def content_type_from_content
-      if @content.respond_to?(:content_type)
-        @content.content_type
-      end
+      @content.meta["content-type"].presence
     end
 
     def filename_from_content_disposition

spec/paperclip/io_adapters/http_url_proxy_adapter_spec.rb

@@ -3,10 +3,9 @@ require 'spec_helper'
 describe Paperclip::HttpUrlProxyAdapter do
   before do
     @open_return = StringIO.new("xxx")
-    @open_return.stubs(:content_type).returns("image/png")
-    @open_return.stubs(:meta).returns({})
-    Paperclip::HttpUrlProxyAdapter.any_instance.
-      stubs(:download_content).returns(@open_return)
+    @open_return.stubs(:meta).returns("content-type" => "image/png")
+    Paperclip::HttpUrlProxyAdapter.any_instance.stubs(:download_content).
+      returns(@open_return)
     Paperclip::HttpUrlProxyAdapter.register
   end
 

spec/paperclip/io_adapters/uri_adapter_spec.rb

@@ -16,6 +16,8 @@ describe Paperclip::UriAdapter do
   end
 
   context "a new instance" do
+    let(:meta) { { "content-type" => "image/png" } }
+
     before do
       Paperclip::UriAdapter.any_instance.
         stubs(:download_content).returns(@open_return)
@@ -71,6 +73,7 @@ describe Paperclip::UriAdapter do
 
   context "a directory index url" do
     let(:content_type) { "text/html" }
+    let(:meta) { { "content-type" => "text/html" } }
 
     before do
       Paperclip::UriAdapter.any_instance.