Bugfix/issue 1121 multivariate error

#### Summary:

Fix #1121 by ensuring that stan::math::multiply_lower_tri_self_transpose returns a symmetric matrix. 

#### Intended Effect:

* Double version of multiply_lower_tri_self_transpose matches the output of the var version of multiply_lower_tri_self_transpose 

#### How to Verify:

* Running `./runTests.py src/test/unit/math/matrix/multiply_lower_tri_self_transpose_test.cpp`. 
* Running the model outlined in #1121 should no longer fail before 100 warmup iterations

#### Side Effects:

None.

#### Documentation:

No changes.

#### Reviewer Suggestions:

Anyone