(PDB-220) Coerce numerical function output in manifests to string

Previously, when a user defined a numeric-valued function in a puppet manifest
and submitted it to notify, the resource-title would remain numeric and
PuppetDB would throw exceptions while storing reports. Per the docs,
resource-title must be a string. This pull request avoids the problem by
coercing resource-title to string.
Merge pull request #994 from wkalt/ticket/master/pdb-220-report-storage-numeric-functions

(PDB-220) Coerce resource-title to string