Allowing PostBody to support multipart/form-data with base64 encoded string

The explode function was not properly getting the key/value from the string.

For example the base64 encoded string "/xA2JhWEqPcgyLRDdir9WSRi/khpb2Lh3ooqv+5VYoc=" was missing the trailing "=" character.

Running the new unit test PostBodyTest::testMultipartWithBase64Fields() without applying the change results in the following error:

```
There was 1 failure:

1) GuzzleHttp\Tests\Post\PostBodyTest::testMultipartWithBase64Fields
Failed asserting that '--53c5c5b905f14
Content-Disposition: form-data; name="foo64"

/xA2JhWEqPcgyLRDdir9WSRi/khpb2Lh3ooqv+5VYoc
' contains "/xA2JhWEqPcgyLRDdir9WSRi/khpb2Lh3ooqv+5VYoc=".

/Users/jamie/ji/guzzle/tests/Post/PostBodyTest.php:168
```