Modifying sourcepath entry takes previous attributes into account

When the output location of a sourcepath entry was modified by
m2e-android, it created a whole new classpath entry with default
values, so properties added to the old entry by m2e was dropped. This
new implementation just modifies the old entry object, so all attributes
added by m2e are retained.

See here for more information: #182
Refactor MavenEclipseClasspath#setClassPathEntryExported

Now we add the attributes of the previous classpath container to
the newly created container which is marked as exported.
Added test to verify classpath attributes are not dropped