Provide MediaType to AbstractJacksonHttpMessageConverter.customizeReader

I am implementing a CSV message converter by extending AbstractJacksonHttpMessageConverter.

While customizerWriter supplies the target MediaType, customizeReader does not supply the source MediaType.

As a result, I cannot check the "header" parameter to decide the value for CsvSchema.withUseHeader.

A new extension point could be added without breaking anything:

protected ObjectReader customizeReader(
        ObjectReader reader, JavaType javaType, @Nullable MediaType contentType) {

    return customizeReader(reader, javaType);
}

This could also be of general use for other formats, as media types such as application/x-my-thing+json are commonly used, and knowing the base type may be useful in configuring a reader.

Start at AbstractJacksonHttpMessageConverter and compare customizeReader with customizeWriter, then inspect the surrounding message-converter tests. Verify how MediaType is passed during reader customization and preserve the existing overload behavior; done means CSV and other media-type-aware readers can access the source content type without breaking existing subclasses.