Might be worth checking which version of esp8266httpclient you are using, because it looks like
some versions had issues with https.
Regarding the garbage, I'd be wanting to find a way to confirm if the garbage is there on the Oak, or just being added by Particle (or on the way to Particle)?
Maybe concatenate two strings together into a new string, then particle.println one after the other the two original strings, and the joined string. See if the garbage remains at the end, or follows into the middle of the merged string, or just occurs once at the end of the merged string. Also does the garbage change between string and concatenated version?
Then next time try changing the order so you print the original two before concatenating and printing joined string. In case the particle.println is corrupting the strings.
Then try doing particle.println for all variables, followed by another set of printing all variables again to see if the printed strings match each other, and if the garbage the 2nd time matches the first time.
These tests should hopefully let you confirm definitively whether the problem is occurring on the Oak and is possibly related to https problems, or whether it is just random garbage being put there by particle. Hopefully it is just Particle, and your strings have integrity on the Oak (eg they are not screwing up you url string etc)
Mike