Multipart: boundary should not be random

Currently boundary is random, so the pact is generated differently each time I run consumer test:

diff --git a/example/multipart/pacts/multipartConsumer-multipartProvider.json b/example/multipart/pacts/multipartConsumer-multipartProvider.json
index eb2fcc3..9dca32f 100644
--- a/example/multipart/pacts/multipartConsumer-multipartProvider.json
+++ b/example/multipart/pacts/multipartConsumer-multipartProvider.json
@@ -11,11 +11,11 @@
         }
       ],
       "request": {
-        "body": "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",
+        "body": "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",
         "headers": {
           "Accept": "application/json",
           "Authorization": "Bearer eyJhbGciOiJIUzI1NiIXVCJ9",
-          "Content-Type": "multipart/form-data; boundary=ktJmeYHbkTSa1jxD"
+          "Content-Type": "multipart/form-data; boundary=gpDtEvot4ma3EDUp"
         },
         "matchingRules": {
           "body": {

Here is how to reproduce it:

git clone https://github.com/pact-foundation/pact-php.git
cd pact-php
git checkout ffi
composer install
cd example/multipart/consumer/
PACT_LOGLEVEL=trace phpunit
Expected: the generated pact file example/multipart/pacts/multipartConsumer-multipartProvider.json should not be changed
Actual: the generated pact file example/multipart/pacts/multipartConsumer-multipartProvider.json is changed because of the dynamic boundary

According to this article this behavior should be avoid.

I have tried several solutions:

Get boundary from Content-Type header
- Code: https://github.com/pact-foundation/pact-reference/compare/master...tienvx:pact-reference:multipart-get-boundary-from-header
- Cons: pactffi_with_header_v2 need to be called with Content-Type before pactffi_with_multipart_file
Create new ffi function pactffi_with_multipart_boundary
- Code (WIP): https://github.com/pact-foundation/pact-reference/compare/master...tienvx:pact-reference:ffi-with-multipart-boundary
- Cons: pactffi_with_multipart_boundary need to be called after pactffi_with_multipart_file
Create new ffi function pactffi_with_multipart_file_v2, similar to pactffi_with_multipart_file but with additional parameter boundary
- Code: https://github.com/pact-foundation/pact-reference/compare/master...tienvx:pact-reference:ffi-with-multipart-file-v2
- Cons: none, I think
Add boundary parameter directly to pactffi_with_multipart_file
- Code: Not available, but similar to https://github.com/pact-foundation/pact-reference/compare/master...tienvx:pact-reference:ffi-with-multipart-file-v2
- Cons: create breaking change

pact-foundation / pact-reference

Multipart: boundary should not be random #322