FPM::Package#convert does not keep defaults attrs

This is a failing spec to show a bug in FPM::Package#convert:
Default attributes of the target package format are applied when calling .new,
but then discarded and entirely replaced with the attributes of the source
package.
FPM::Package#convert now maintains default attrs

FPM::Package#convert no longer discards the default values of the attributes of
the target package.
Instead the attributes of the source package are merged into the default
attributes of the target package.
Merge pull request #538 from jkanywhere/convert-with-default-attrs

Convert with default attrs