Remove some deprecate warnings

Adding the default rcParams to itself will also include some deprecated
params which then show up as deprecated warnings. Remove such params
before adding them.

Also added is a better version to check for matplotlib bug #2543: the
new workaround will only be triggered for the problematic
'text.dvipnghack' param and leave all other params alone. This was
needed because some params accept None but not "none"...