Add lenient mode to MediaType parser

Users often find it surprising that methods like message.contentType()
can throw IllegalArgumentException when the media type is invalid. So
this adds a MediaType.parseLeniently() method that tries to recover from
 invalid media type strings and uses it in such cases.

Fixes #83

src/org/netpreserve/jwarc/GeminiResponse.java

@@ -99,7 +99,7 @@ public MediaType contentType() {
             if (meta.isEmpty()) {
                 return MediaType.parse("text/gemini; charset=utf-8");
             }
-            return MediaType.parse(meta);
+            return MediaType.parseLeniently(meta);
         } else {
             return MediaType.OCTET_STREAM;
         }