fix: mime type string deprecation

mime-types 3.6.0 warns "MIME::Type.MIME::Type.new when called with a String is deprecated."

change introduced in and syntax borrowed from:
mime-types/ruby-mime-types@8512c15

lib/mechanize/pluggable_parsers.rb

@@ -104,7 +104,7 @@ def parser content_type
 
     return parser if parser
 
-    mime_type = MIME::Type.new content_type
+    mime_type = MIME::Type.new "content-type" => content_type
 
     parser = @parsers[mime_type.to_s] ||
              @parsers[mime_type.simplified] ||