Handle spaces better in for loop expressions

[jasmith-hs]

The logic for "For loop" expressions is pretty wonky. Let's clean it up a bit and allow Jinjava to support for loops with maps that contain spaces. For example:

{% for item in [{'key': 'foo?'}, {'key': 'bar?'}] %}

Currently this gets transformed into:

{% for item in [{'key':,'foo?'},{'key':,'bar?'}] %}

which is invalid syntax. (Notice the commas after the colons)
This PR makes that not happen by never splitting up the expression. Instead we look for " in " and treat everything after that as the loop expression. The previous logic did this, but in a more convoluted way by first splitting up all the helpers (treating spaces and commas the same way) and then rejoining them with commas.