Move from using a char prefix to string prefix
removed default null character from mcmc_writer and replaced with empty string
fixed issue with mcmc_writer_test using char rather than string
Updated signature of test implementation to restore valid compilation
Merge branch 'feature/issue_394' of https://github.com/stan-dev/stan into feature/issue_394
fixed failing test
Merge branch 'develop' into feature/issue_394
Merge pull request #504 from stan-dev/feature/issue_394

Fixes #394.