convert: handle non-string inputs gracefully

[yaauie]

• convert literal-true and literal-false to 1.0 and 0.0 respectively
• strip commas only if value is a string
• share implementation details with convert_float_eu

Fixes: logstash-plugins/logstash-filter-mutate#120 Supersedes: logstash-plugins/logstash-filter-mutate#121

[jsvd]

@yaauie do you mind bumping the patch version and updating the changelog before merging?