Reusing SSLContext in a session

[echo4eva]

A pull request to take a hit at fixing issue #4322

Worked with my team and we made it so that HTTPAdapter has a default SSLContext initialized when the adapter is constructed. SSLContext is made by using urllib3's function to create context. The context is passed into the poolmanager as pool_kwargs which can take in SSLContext.

Tested the reuse of SSLContext by using jmcackle's test functions they've made for adapters.