Investigate supporting the Expect header in RPC

[cryptocode]

curl (and curl wrappers) sends the Expect header by default. For large requests (>1K), this causes curl to delay for 1 second waiting for the "100-continue" response. Postman and many other clients does not send this by default.

A workaround for curl is to add -H Expect:

Adding support for this header in the node may be unnecessary if RPC gets externalized, as any external web server is likely to support this already.