Rename `sendPOST` to `sendPost`

[ThomasLandauer]

Reasons:

• Consistency with lib-innerbrowsers sendAjaxPostRequest(), see e.g. https://codeception.com/docs/modules/Symfony#sendAjaxPostRequest
• PSR-1 compliance:

  Method names MUST be declared in camelCase

And, of course, the same for sendGET, sendDELETE, etc.

Suggested upgrade path, according to semantic versioning:

1. Create sendPost in next minor version (i.e. v1.3)
2. Keep sendPOST as alias, but deprecate it.
3. Remove sendPOST in next major version (i.e. v2.0)

[SamMousa]

Actually, in PHP function names are not case sensitive.
So we can just rename the function and that is not a break in BC.