Support IO-like objects efficiently

Previously if we passed in an IO object which doesn't respond to #seek,
MimeMagic would read the whole IO into memory. And this is exactly the
type of objects that the Shrine file upload library deals with.

However, we can also "seek" by blank-reading that many bytes, and always
rewinding to the beginning, which is what we've implemented in this
change.

This change also doesn't require IOs to respond to #binmode.

lib/mimemagic.rb

@@ -109,28 +109,27 @@ class MimeMagic
   end
 
   def self.magic_match(io, method)
-    if io.respond_to?(:seek) && io.respond_to?(:read)
-      io.binmode
+    return magic_match(StringIO.new(io.to_s), method) unless io.respond_to?(:read)
+
+    io.binmode if io.respond_to?(:binmode)
     io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)
     buffer = "".force_encoding(Encoding::BINARY)
+
     MAGIC.send(method) { |type, matches| magic_match_io(io, matches, buffer) }
-    else
-      str = io.respond_to?(:read) ? io.read : io.to_s
-      magic_match(StringIO.new(str), method)
-    end
   end
 
   def self.magic_match_io(io, matches, buffer)
     matches.any? do |offset, value, children|
       match =
         if Range === offset
-          io.seek(offset.begin)
+          io.read(offset.begin, buffer)
           x = io.read(offset.end - offset.begin + value.bytesize, buffer)
           x && x.include?(value)
         else
-          io.seek(offset)
+          io.read(offset, buffer)
           io.read(value.bytesize, buffer) == value
         end
+      io.rewind
       match && (!children || magic_match_io(io, children, buffer))
     end
   end

test/mimemagic_test.rb

 require 'bacon'
 require 'mimemagic'
 require 'stringio'
+require 'forwardable'
 
 describe 'MimeMagic' do
   it 'should have type, mediatype and subtype' do
@@ -116,12 +117,15 @@ describe 'MimeMagic' do
 
   it 'should handle different file objects' do
     MimeMagic.add('application/mimemagic-test', magic: [[0, 'MAGICTEST']])
-    class ReadableObj
-      def read
-        'MAGICTEST'
+    class IOObject
+      def initialize
+        @io = StringIO.new('MAGICTEST')
       end
+
+      extend Forwardable
+      delegate [:read, :size, :rewind, :eof?, :close] => :@io
     end
-    MimeMagic.by_magic(ReadableObj.new).should.equal 'application/mimemagic-test'
+    MimeMagic.by_magic(IOObject.new).should.equal 'application/mimemagic-test'
     class StringableObject
       def to_s
         'MAGICTEST'