Update RelaxedDataBinder so that both `.` and `_` are considered in
getPropertyValuesForNamePrefix(...).

With Spring Boot 1.2.5 binding environment variables of the form
`FOO_BAR_BAZ` to `@ConfigurationProperties(prefix="foo-bar")` objects
worked thanks to a happy accident. When `PropertySourcesPropertyValues`
processed a non enumerable `PropertySource` it called the resolver
with a property name `FOO_BAR.BAZ`. A `SystemEnvironmentPropertySource`
will replace `.` with `_` and hence find a value.

Commit 1abd0879 updated non enumerable processing such that the resolver
was never called.

Replicating the problem is quite involved as you need to ensure that you
have both a SystemEnvironmentPropertySource and a non-enumerable
property source (e.g. RandomPropertySource). A test has been added to
PropertiesConfigurationFactoryTests which passes on 1.2.5, fails on
1.2.6 and passes again following this commit.

Fixes gh-4045