HTTP/2 throw statement missed

Motivation:
The DefaultHttp2FrameWriter has an exception generated but is missing the throw keyword.

Modifications:
Insert the missing throw keyword.

Result:
Exception thrown when it was intended to be thrown.
HTTP/2 Prohibitied Cihpers Allowed

Motivation:
The Http2SecurityUtil class lists a few ciphers that are explicitly prohibited by the HTTP/2 specification because of their characteristics.

Modifications:
Remove the ciphers that are prohibited.

Results:
Cipher suite used for HTTP/2 codec is compatible with HTTP/2 spec.
Remove or de-prioritize RC4 from default cipher suites

Motivation:

RC4 is not a recommended cipher suite anymore, as the recent research
reveals, such as:

- http://www.isg.rhul.ac.uk/tls/

Modifications:

- Remove most RC4 cipher suites from the default cipher suites
- For backward compatibility, leave RC4-SHA, while de-prioritizing it

Result:

Potentially safer default