Fixed cache.py bug and test

[cdelafuen]

Related to issue #49

Removed the "long" from the format of master_id, so now can support not integer values for primary_keys.

[coveralls]

Coverage remained the same when pulling de3bb0f on cdelafuen:patch-1 into 27949ed on edoburu:master.

[mzaforas]

I have detected this issue in my proyect also, PK as integers is a very restrictive condition.
I support the change!

Thanks

[IvanAlegre]

I use string pks in most of my classes so this fix is needed

[aleonsan]

I strongly support the change.
We cant assume that master pk should be casted as a specific type. LGTM

[redondofelix]

Thanks @cdelafuen for your PR. I sent an email weeks ago to fix this issue because is critical in my project. Please merge it.

[NotSqrt]

You'll probably need to add tests on this, to be sure that everything is good !

[mdogo]

I have the same problem in one of my projects. I do not understand why to cast the primary key to a long type.

[Hispar]

This solved the issue in our project. Thanks!

[coveralls]

Coverage increased (+0.07%) when pulling 9ed8253 on cdelafuen:patch-1 into 27949ed on edoburu:master.

[cdelafuen]

Added test to cover this bug.