Add some constructors or setters

[smallhive]

Writing unit tests requires creating the following structures, to override rpc calls from client:

• rpc.PutRequestWriter
• rpc.GetResponseReader
• rpc.ObjectRangeResponseReader
• rpc.SearchResponseReader

The problem is, these structures contain private fields of public types and there is no way (neither constructor nor setter) to set them up outside api-go. Without them, some client methods can't be tested without real node