allow changing the VERSION constant

[nickshek]

This pull request allows to overwrite the default version in http requests by subclassing. This can be useful to when the developer using this composer package want to control the version number.

[philipbrown]

Why would you want to change the version number? The version number is supposed to track the version number of the package, and so IMO it would only get bumped when an update to the package is released.

I'm curious to hear what your use case is?

[nickshek]

Suppose we want to develop a API like this:

http://mydomain.example/get_post.php?post_id=xxxx&api_version=xxxx&
api_key=xxxx&api_timestamp=xxxx&api_signature=xxxx

If we want to use this package for developing this api,there will be two problems:

1. api_version must be presented
2. API developers are not able to control the version number

[philipbrown]

Why would you want to change the version number? The version number is used to ensure that both client and server are using the same version of the package. So the version number would only change when the package was updated.

This version number has nothing to do with your API version number.