Propagators should support all forms of Instana Headers

[pglombardo]

Depending on where Inject and Extract are being called from, the HTTP headers can vary in capitalization and prefix.

For example, in Flask and Django, in the middleware, HTTP headers are presented as all upper case, underscores and prefixed with HTTP_ (Ruby also does this in Rack):

• HTTP_X_INSTANA_T

Yet in making requests via the requests package, HTTP headers are in the on-the-wire format:

• X-Instana-T

Our propagators should support all variations:

• Capitalization
• Underscores versus dashes
• Prefixed or not (only support HTTP_ for now)

[pglombardo]

With this PR, we support Extract as X-Instana-[TS] and the alternate form HTTP_X_INSTANA_[TS].

For Inject, we only support Inject with X-Instana-[TS] as OpenTracing has no mechanism to signal otherwise.