[Issue-370] - Update createTestDoublesForFunctionNames to return an object

[bannmoore]

cc @primarilysnark

This PR addresses Issue 370.

Context: When td.object is called with an array of function names, it returns an array; my understanding of the issue is that td.object should always return an object.

From the original issue:

> td.object({ something() {} }).splice
undefined
> td.object(['something']).splice
[Function: splice]

Included Changes:

• Add an assertion to the td.object w/ array of function names test, to confirm that the resulting object is not an array.
• Add a base value of {} to the accumulator in createTestDoublesForFunctionNames, to ensure that it responds with an object.

[primarilysnark]

Looks great. Thanks for addressing this annoying error. 👍

[searls]

Landed in v3.12.1