Call relation methods in the right context

Before this change, it was impossible to override 'build' and other
(custom) relation/scope methods; only the original prototype method was
ever called.
Merge tag 'v2.10.0'

2.10.0
Enable include scope for belongsTo
Merge pull request #327 from fabien/fix/call-scope-method

Call relation methods in the right context
Merge pull request #329 from fabien/feature/include-scope

Enable include scope for belongsTo
Bump version
Merge branch 'release/2.10.1' into production