add default prefix and allow prefixes with spaces

This is a change I would like peer-reviewed. If the default strategy changes are rejected, I can submit a separate request to just allow prefixes with spaces.
Allows you to use the default strategy (match key attributes) when a '=' is in the locator.
Locator `page?a=b` will still cause error, but can be changed to `default=page?a=b`.
Locator prefixes will now be space-insensitive so the following will work when the previously did not:
* `partiallink=text`
* `x path = //div`

Fixes #193
Good catch! That was copypasta on my part - the unit test was broken before my changes. It looked funny to me too, but my mind was elsewhere. Decided to use the utility method due to shortcomings in 2.6 you mentioned. More readable than standard catch. Thanks!