A blank string used for placeholder replacement can cause memory issue

[pjbollinger]

Version: betamax-0.8.0

If you define a placeholder as described in the following, it can lead to memory issues, depending on how many blank strings you're trying to replace.

username = ''
config.define_cassette_placeholder('<USERNAME>', username)

For example, if I have the original string 'WOW' and perform the .replace() function, it will result in the following.

>>> 'WOW'.replace('','<USERNAME>')
'<USERNAME>W<USERNAME>O<USERNAME>W<USERNAME>'

If you have multiple placeholders with a blank string, this compounds and can cause a memory issue. I experienced this issue in src.betamax.cassette.interaction.py::Interaction::replace_in_uri, but would think this is applicable to all functions that use str.replace() and it looks like it exists on the most current version of betamax too.

I can submit a PR to check for blank strings prior to calling str.replace(), but figure I'd bring it up in an issue first.

[hroncok]

Since emoji responses don't send notifications, I'm letting you know I like the idea to check for blank strings and refusing to continue.