Fix JRUBY-6731

Can't load class files in WARs compiled by warbler using jruby 1.7.0.preview1

When requiring a file with an explicit .class extension, assume
the .class filename is the only filename to be searched.

Because a require of .class almost always will indicate that the
user wants to load a specific precompiled Ruby file, there's no
good reason (I know of) to search for anything else.

The original logic did not work at all; require 'foo.class' would
search for 'foo.class.rb' and 'foo.class.class' (in that order)
but never 'foo.class'.

The claim is that this worked in 1.6.7.2, but I could not
reproduce that with a simple case. The logic involved does not
appear to have changed in a substantial way since well before
1.6.7.2, and in any case I believe the fix here is the right way
for it to work.

src/org/jruby/runtime/load/LoadService.java

@@ -685,6 +685,10 @@ public void prepareRequireSearch(final String file) {
 
                     // trim extension to try other options
                     searchFile = file.substring(0, matcher.start());
+                } else if (file.endsWith(".class")) {
+                    // For JRUBY-6731, treat require 'foo.class' as no other filename than 'foo.class'.
+                    suffixType = SuffixType.Neither;
+                    searchFile = file;
                 } else {
                     // unknown extension, fall back to search with extensions
                     suffixType = SuffixType.Both;