Added expectBodyLength function for all content types

[gwcgm1]

Had a go at adding this function - found a use for it ourselves but might not be so useful to others.

[Fishbowler]

Thanks for the contribution @gwcgm1!
Any chance you could add some unit tests for this?
There's some good examples of passing/failing expects based on body around here: https://github.com/IcedFrisby/IcedFrisby/blob/master/test/frisby_mock_request.js#L1135

[Fishbowler]

I've been looking at some similar missing coverage in Coveralls, and I'm starting to believe that the npm request package used under the hood guarantees body to never be null once you're running expects on the response.

The only way I can see to exercise that null check / error throwing line would be to set a mock in the params, and have that mock function return a null body - a faff too far for the scope of this.

[Fishbowler]

I've removed the need for this check in the unit-test-coverage branch. Definitely don't worry about it. In fact, you can remove the entire isUndefined check - as I said, it can't occur without mocking.

[Fishbowler]

I've removed that check (included in PR #124, merging soon), and had a punt at some unit tests.
I'm convinced of coverage, but might be a touch extreme, and might need clearer wording too!

[paulmelnikow]

If the operator is required, would it be a little clearer to put it before the number? Then this reads like an expression: "expect body length greater than 418".