Fixing [excon][WARNING] Invalid Excon request keys log noise when trying...

Currently when trying to use assume_role, fog generates the following noisy warning.  It doesn't affect the operation of the request, but does clog up the output.

```
[excon][WARNING] Invalid Excon request keys: :host
/Library/Ruby/Gems/2.0.0/gems/excon-0.31.0/lib/excon/connection.rb:389:in `validate_params'
/Library/Ruby/Gems/2.0.0/gems/excon-0.31.0/lib/excon/connection.rb:225:in `request'
/Library/Ruby/Gems/2.0.0/gems/fog-1.20.0/lib/fog/xml/sax_parser_connection.rb:36:in `request'
/Library/Ruby/Gems/2.0.0/gems/fog-1.20.0/lib/fog/core/deprecated/connection.rb:18:in `request'
/Library/Ruby/Gems/2.0.0/gems/fog-1.20.0/lib/fog/aws/sts.rb:121:in `request'
/Library/Ruby/Gems/2.0.0/gems/fog-1.20.0/lib/fog/aws/requests/sts/assume_role.rb:31:in `assume_role'
```