[Form]fixed FormRenderer::humanize() to humanize camel cased label

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

FormRenderer::humanize() only converts underscored_field_name to humanized field name(just the same as sfInflector::humanize()).
Sensio\Bundle\GeneratorBundle\Generator\DoctrineFormGenerator does not convert camelCased field names to underscored one, however.
I have to edit manually field names in the generated FormType class everytime I use doctrine:generate:form Command, too messy.
so I suggest humanize() is to take care of it.