Adding 'wiki' representation for 'convert_contentbody_to_new_type'.

[kymikoloco]

The Confluence REST API seemingly supports converting from wiki format to (at least) their storage format. However, the function convert_contentbody_to_new_type disallows old_representation to be 'wiki', as it looks like it was taken from their docs directly.

I have tested this locally with 2.7.10 and 3.5.2, and on my Confluence Server 5.10.1 it correctly returns the storage format.

See the Confluence Dev Content Conversion examples for the suggested usage.