Merge custom headers with defaults rather than existing headers

[Shaptic]

Closes #1088

[github-actions]

Size Change: +72.3 kB (+0.15%)

Total Size: 49.1 MB

Filename	Size	Change
dist/stellar-sdk-minimal.js	6.76 MB	+10.4 kB (+0.15%)
dist/stellar-sdk-minimal.min.js	5.28 MB	+7.64 kB (+0.14%)
dist/stellar-sdk-no-axios.js	6.76 MB	+10.4 kB (+0.15%)
dist/stellar-sdk-no-axios.min.js	5.28 MB	+7.63 kB (+0.14%)
dist/stellar-sdk-no-eventsource.js	7.01 MB	+10.4 kB (+0.15%)
dist/stellar-sdk-no-eventsource.min.js	5.49 MB	+7.63 kB (+0.14%)
dist/stellar-sdk.js	7.01 MB	+10.4 kB (+0.15%)
dist/stellar-sdk.min.js	5.49 MB	+7.63 kB (+0.14%)

_{compressed-size-action}