Attempt to clear the BufferedReader class from IndexError

[stankudrow]

Addresses the following issues:

• Bug IndexError #69
• IndexError: bytearray index out of range #25
• [BUG] Stale connection in connection pool cannot properly recover itself #20
• Exception #10

Continues the PR #90.

The assumption is the case when len(self.buffer) == self.position. Currently only two cases were caught:

1. self.buffer=b"" and self.position = 0 - can be when realising the logic of data reading;
2. when self.buffer is not empty and self.position = len(self.buffer) -> no reproducible example to lay out.

During the investigation, some refactoring stuff was brought.

[stankudrow]

@long2ice , hello, could you give your opinion on the PR and provide answers to the comments on the test cases described in the docstrings?

Also, could you tell are you planning to release a new version with, at least, this #90 PR to be included?

[stankudrow]

@long2ice

[stankudrow]

@long2ice , could you have a look at this PR and tell whether it is OK to be merged?

[long2ice]

Thanks!