changed code to use default branch if none is specified.

[Armaxxx]

Resolves: python-poetry/poetry#3366

• Added tests for changed code.
• Updated documentation for changed code.

[Armaxxx]

So, the tests are failing because they expect "master" to be the default branch, however by removing the branch from the url the default "main" branch will be pulled, however it is named.

[finswimmer]

Hello @Armaxxx,

I just took a quick look at this. I think at least this method

poetry-core/poetry/core/packages/vcs_dependency.py

Line 97 in abe9fe5

def base_pep_508_name(self): # type: () -> str

must be adopted, to not add a branch if there is no one defined.

fin swimmer

[Armaxxx]

You're right, I completely missed it, adapted.

[kasteph]

Closing in favor of #143. Thank you @Armaxxx.