Revert |f behavior for 0.7.1.

This reverts the behavior of the `|f` helper to what it was before b40ccfcba362092a79001aa5c0da77fd904ed692, per the conversation in #54. Also adds tests to prevent regressing the behavior of either `|f` or `|fe`.

Fixes #54.
typo? `Always converts to text type before interpolation`.
I'd also love a comment here saying that this will "unsafe" things that may have been "safed" before: a Markup (safe) object will be converted to an (unsafe) string.