fix for content metadata key conflict

[jzyinq]

Fixes #96

[eyeseast]

Sorry to let this sit so long. I appreciate it.

I tested this locally, and there's another issue, which is my fault: There are two ways to serialize the resulting post to a dictionary, and they give different results.

---
content: bad key
---

Ooops!

>>> import frontmatter
>>> p = frontmatter.load('oops.md')
>>> p.content
'Ooops!'
>>> dict(p)
{'content': 'bad key'}
>>> p['content']
'bad key
>>> p.metadata
{'content': 'bad key'}
>>> p.to_dict()
{'content': 'Ooops!'}

So that's a problem, and it needs a decision more than a technical solution. No matter what, it's going to be a breaking change somewhere.

[jzyinq]

I understand, but do you plan to release a new major version then? It seems that in the end, making new major with instruction how to migrate will be beneficial for many ppl anyway.

I see that many forks are created just to add that fix.

[eyeseast]

I will push a new release soon, just haven't had time yet with work stuff. I need to decide on this one and pull together any other breaking changes.