Support the -Wassign-enum compiler flag. (#2085)

Support the -Wassign-enum compiler flag.
diff --git a/objectivec/GPBCodedOutputStream.m b/objectivec/GPBCodedOutputStream.m
index 251a159..7c3ab44 100644
--- a/objectivec/GPBCodedOutputStream.m
+++ b/objectivec/GPBCodedOutputStream.m
@@ -290,7 +290,7 @@
                      maxLength:bufferBytesLeft
                     usedLength:&usedBufferLength
                       encoding:NSUTF8StringEncoding
-                       options:0
+                       options:(NSStringEncodingConversionOptions)0
                          range:NSMakeRange(0, [value length])
                 remainingRange:NULL];
     }