Support http-request with multipart/form-data.

When sending http-request using curl or postman with content-type as ‘application/json’, it is working well.
`curl -XPOST "http://localhost:9000/2015-03-31/functions/function/invocations" -d '{[body with json]}'`
But with content-type as ‘multipart/form-data’, it is not working with error about “unmarshal input : ‘utf-8’”
`curl -XPOST "http://localhost:9000/2015-03-31/functions/function/invocations" --form '[some message or file]'`

Error log
`START RequestId: 869477bd-4c63-4d81-802c-416e9634067b Version: $LATEST
Traceback (most recent call last):able to unmarshal input: 'utf-8' codec can't decode byte 0xb9 in position 451: invalid start byte
END RequestId: 869477bd-4c63-4d81-802c-416e9634067b
REPORT RequestId: 869477bd-4c63-4d81-802c-416e9634067b Duration: 20.28 ms Billed Duration: 100 ms Memory Size: 3008 MB Max Memory Used: 3008 MB`

It need to support content-type as ‘multipart/form-data’. I wonder it is possible or only support ‘application/json’.

首先，使用 issue 中的 curl --form 範例重現對 /2015-03-31/functions/function/invocations 的請求，並與正常運作的 JSON 請求進行比較。從該 endpoint 開始追蹤 emulator 的請求處理流程；當包含 messages 或 files 的 multipart/form-data 請求能夠被接受，且不再出現 UTF-8 unmarshal 錯誤時，即表示完成。

由索引模型根據 Issue 內容生成。