Update koa-bodyparser to ^4.2.1 (Fixes content-length mismatch)

[brendanmoore]

Issue:

apollo-server-koa will incorrectly throw request size did not match content length from raw-body dependency when a well form request with content-length header is followed by well formed request without a content-length header i.e. (chunked encoding)

Cause:

koa-bodyparser creates singleton options objects for co-body on this line, the version of co-body that Apollo server koa resolves to will mutate that options object fixed in this commit when content-length header is present the value is persisted in the singleton. So any follow-up request that might not contain content-length will use the previous value.

Reproduction

1. Make any request to apollo-server-koa with a valid body and correct content-length header.
2. Make another request to apollo-server-koa with transfer-encoding: chunked with a different body so the resultant length is different to the first request
3. request size did not match content length will be thrown

Fix:

The co-body dependency has been fixed (in 2017!). Updating the koa-bodyparser to a version that supports the fix will resolve the issue.

[apollo-cla]

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