Fix header files inclusion so that the library compiles

The header files were included using include_lib directive with relative
path; no simple include is used.
Add tests for disabling splitting messages into multiparts

Some controller do not understand multipart messages. Thus ofp_client
reads no_multipart flag and if it's set to true it doesn't split messages
into multiparts.

NOTE: The tests are to be refactored.
Do not split messages into multiparts if explicitly disabled

Some controller do not understand multipart messages. Thus ofp_client
reads no_multipart flag and if it's set to true it doesn't split messages
into multiparts.