null string deserialization typecheck fix

[tchaloupka]

Just a simple exception to pass Json.Type.null_ as a string too as string can be set to null in the json data.

Without this I get this error when not used with the Nullable in the REST API:

Exception while handling request POST /api/v1/events/iot/temp: vibe.http.common.HTTPStatusException@/home/tomas/.dub/packages/vibe-d-0.8.2/vibe-d/web/vibe/web/rest.d(1333): Got JSON of type null_, expected string.
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/usr/include/dmd/phobos/std/exception.d:556 pure @safe bool std.exception.enforce!(bool).enforce(bool, lazy object.Throwable) [0xd1bc30]
/home/tomas/.dub/packages/vibe-d-0.8.2/vibe-d/http/vibe/http/common.d:161 @safe bool vibe.http.common.enforceHTTP!(bool).enforceHTTP(bool, vibe.http.status.HTTPStatus, lazy immutable(char)[], immutable(char)[], int) [0xd33cd7]
/home/tomas/.dub/packages/vibe-d-0.8.2/vibe-d/http/vibe/http/common.d:169 @safe bool vibe.http.common.enforceBadRequest!(bool).enforceBadRequest(bool, lazy immutable(char)[], immutable(char)[], int) [0xd7d734]
/home/tomas/.dub/packages/vibe-d-0.8.2/vibe-d/web/vibe/web/rest.d:1333 @safe void vibe.web.rest.jsonMethodHandler

but I'm not sure if this is ok to add or even enough. What with structs/classes or arrays in the same scenario?