Header.SetExtension does not add an extension with id > 14 as a two-byte extension

[enobufs]

Your environment.

• Version: <=v1.8.13
• Browser: n/a

What did you do?

Add an extension with payload with size <= 16 and extension map ID > 14.

What did you expect?

Header's ExtensionProfile should be set to ExtensionProfileTwoByte.

What happened?

It is set to ExtensionProfileOneByte. Consequently, the receiver (browsers) ignores the extension because its ID is invalid.

[enobufs]

I will create a PR that would fix this (#255) and #249 together.