Rename encode/decode to serializeToString/mergeFromString (#2795)

This better shows the semantic of the API. For already setted fields,
mergeFromString do replacement for singular fields and appending for
repeated fields.
diff --git a/php/tests/proto/test.proto b/php/tests/proto/test.proto
index c971df2..e5dee0f 100644
--- a/php/tests/proto/test.proto
+++ b/php/tests/proto/test.proto
@@ -100,6 +100,7 @@
 enum TestEnum {
   ZERO = 0;
   ONE  = 1;
+  TWO  = 2;
 }
 
 message TestPackedMessage {