oneof_type prevents interpretation of other keyword tags

[Rosswell]

because oneof_type doesnt write to the Type property of a Schema object, the switch statement in structKeywordsFromTags doesn't interpret any other tags. so a set of tags like

Field1 *string `jsonschema:"oneof_type=string;null,enum=VAL1,enum=VAL2"`

will not parse the enum tags, because oneof_type sets the OneOf property instead of the Type property

in this particular example, its arguable that the enum values should not be parsed, but imo if null was an enum value as well i would say that they should be