fix(items): dont override user supplied parameters when updating items

[jgravois]

AFFECTS PACKAGES:
@esri/arcgis-rest-items

ISSUES CLOSED: #117

to be honest, mixing in a properly serialized item like this seems a little strange.

requestOptions.params = {
    ...requestOptions.params,
    ...serializeItem(requestOptions.item)
  };

[coveralls]

Coverage remained the same at 100.0% when pulling fc55db0 on patch117 into 52caf83 on master.

[jgravois]

added additional fixes (and tests) for

• addItemJsonData()
• getItemResources()
• updateItemResource()
• removeItemResource()

[noahmulfinger]

@jgravois searchItems still needs fixing, but it looks like that's the last one.

[noahmulfinger]

Talked with @jgravois, searchItems is fine. LGTM!