fix(traffic-split): upstream_obj.upstream should not be a string

[shreemaan-abhishek]

Description

During creation of roundrobin object for upstream in traffic-split. upstream_obj.upstream: 1 is assigned a string if the upstream configuration only has the weight value.

In this case, if we reload the plugin via the plugin reload API. The roundrobin object gets recreated but since upstream_obj.upstream has a string value. upstream_obj.upstream.vid = i: 2 causes panic because it expects upstream_obj to be a table.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)