Fix issue with Content-Length header being calculated incorrectly - #28

As mentioned in #25, the Content-Length header is not calculated correctly for strings containing certain unicode characters. Here's a summary of the issue:

… the issue here is that the blockscore-node package is relying on String.length to determine the Content-Length header but the header needs to be the byte length, not string length, e.g. 'Zürich' is 7 bytes, not 6. This package should use Buffer.byteLength() instead of String.length to determine the content length header.

For example, this request fails: (note Content-Length: 345)

curl -X POST https://api.blockscore.com/candidates \
  -u "$BLOCKSCORE_API_KEY:" \
  -H 'Accept: application/vnd.blockscore+json;version=4' \
  -H 'Content-Type: application/json' \
  -H 'Content-Length: 345' \
  -d '{"birth_day":"xx","birth_month":"xx","birth_year":"xxxx","document_value":"Fxxxxxxx","document_type":"passport","name_first":"First","name_middle":"","name_last":"Last","address_street1":"Weststrasse xxx","address_city":"Zürich","address_subdivision":"ZH","address_postal_code":"xxxx","address_country_code":"CH","phone_number":"00xxxxxxxxxxxx"}'

with the response,

{
  "error": {
    "type": "invalid_request_error",
    "message": "Poorly formed JSON request body."
  }
}

while this request succeeds: (note Content-Length: 346)

curl -X POST https://api.blockscore.com/candidates \
  -u "$BLOCKSCORE_API_KEY:" \
  -H 'Accept: application/vnd.blockscore+json;version=4' \
  -H 'Content-Type: application/json' \
  -H 'Content-Length: 346' \
  -d '{"birth_day":"xx","birth_month":"xx","birth_year":"xxxx","document_value":"Fxxxxxxx","document_type":"passport","name_first":"First","name_middle":"","name_last":"Last","address_street1":"Weststrasse xxx","address_city":"Zürich","address_subdivision":"ZH","address_postal_code":"xxxx","address_country_code":"CH","phone_number":"00xxxxxxxxxxxx"}'

Closes #25.