Parse out Ghostscript warnings in identfication (#522)

Image info sometimes parses incorrectly because Ghostscript may output
warnings about the validity of the object, despite successfully reading
its attributes. This causes parsing problems because the warning
messages are concatenated with the attributes, and trying to access
these attributes via MiniMagick results in type errors.

The fix is to identify if the warning messages appear, and if so, match
the attributes via strict regular expression.

lib/mini_magick/image/info.rb

@@ -42,7 +42,7 @@ def clear
 
       def cheap_info(value)
         @info.fetch(value) do
-          format, width, height, size = self["%m %w %h %b"].split(" ")
+          format, width, height, size = parse_warnings(self["%m %w %h %b"]).split(" ")
 
           path = @path
           path = path.match(/\[\d+\]$/).pre_match if path =~ /\[\d+\]$/
@@ -61,6 +61,16 @@ def cheap_info(value)
       rescue ArgumentError, TypeError
         raise MiniMagick::Invalid, "image data can't be read"
       end
+
+      def parse_warnings(raw_info)
+        return raw_info unless raw_info.split("\n").size > 1
+
+        raw_info.split("\n").each do |line|
+          # must match "%m %w %h %b"
+          return line if line.match? /^[A-Z]+ \d+ \d+ \d+B$/
+        end
+        raise TypeError
+      end
 
       def colorspace
         @info["colorspace"] ||= self["%r"]