Add log_headers option to globals

[EslamMoh]

What kind of change is this?

Feature. This makes logging request and response headers optional

Did you add tests for your changes?

Added unit tests for options_spec

Summary of changes

Motivation: Savon log headers which contain Authorization header most of the time and I think it will be better for more security to hide this header from logs or at least add an option to give the developer the freedom to log it or not.

Changes: I did that by adding log_headers option to the GlobalOptions class and giving it a default value true, so the current users won't be affected.

Other information

[pcai]

@EslamMoh thank you for your contribution!