Major Description
JacksonUtils.verifySchema verifies if a data type can be converted to JSON. For MapType, it now verifies the key type. However, in JacksonGenerator, when converting a map to JSON, we only care about its values and create a writer for the values. The keys in a map are treated as strings by calling toString on the keys.
Thus, we should change JacksonUtils.verifySchema to verify the value type of MapType.
