msgpack: fix a bug that float32 value isn't be unpacked correctly
msgpack: fix a bug that float32 value isn't be unpacked correctly

MSGPACK_OBJECT_FLOAT32_VALUE(element) returns a double value not a
float value.

komainu8/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga