protoc_plugin: Improve "is map field" check (#765)
protoc_plugin: Improve "is map field" check (#765)

`MessageOptions.map_entry` is an `optional bool` field. Currently to
check whether a field is a map we check whether `map_entry` is present,
but we don't check its value. This only works if the field is never set,
or set to `true` (i.e. it's never set to `false`).

The right way to check it is

    descriptor.options.hasMapEntry() && descriptor.options.mapEntry

because the default value of the field is `false`, this can be
simplified as

    descriptor.options.mapEntry
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