Remove `org.apache.http.HttpHeaders` dependency in `OkHttpClientAdapter`

[paulboocock]

Describe the bug
We still import in org.apache.http.HttpHeaders within OkHttpClientAdapter, however OkHttp no longer imports HttpClient like it used to (a long time ago!).

snowplow-java-tracker/src/main/java/com/snowplowanalytics/snowplow/tracker/http/OkHttpClientAdapter.java

Line 29 in d60f3f7

import org.apache.http.HttpHeaders;

We don't actually need to use this import, we can just use the string values for the HttpHeader values.

snowplow-java-tracker/src/main/java/com/snowplowanalytics/snowplow/tracker/http/OkHttpClientAdapter.java

Line 134 in d60f3f7

.addHeader(HttpHeaders.CONTENT_TYPE, Constants.POST_CONTENT_TYPE)

[mscwilson]

The .addHeader call on line 135 was .addHeader("Content-Type", Constants.POST_CONTENT_TYPE) originally, up until this bug fix #195. I'm not sure why that particular change - "Content-Type" to HttpHeaders.CONTENT_TYPE was made.

Edit: technically the first version was .addHeader("Content-Type", "application/json; charset=utf-8")

[paulboocock]

The .addHeader call on line 135 was .addHeader("Content-Type", Constants.POST_CONTENT_TYPE) originally, up until this bug fix #195. I'm not sure why that particular change - "Content-Type" to HttpHeaders.CONTENT_TYPE was made.

Edit: technically the first version was .addHeader("Content-Type", "application/json; charset=utf-8")

I'm not sure why I did it either. I think I likely just copy and pasted and my IDE auto imported it. Either way, going back to the string is the correct call.