Fix parsing of already-decoded URIs

[zikolach]

I have issue with UriInterpolator attempting to decode part of URL which which is not originally encoded (just contains % character) or was already decoded.

Sample URL
https://storage.googleapis.com/realtyplusmag-news-photo/news-photo/108462.Welspun-Set-To-Acquire-70%-Debt-Of-Sintex-BAPL-2.jpg

Stack trace

URLDecoder: Illegal hex characters in escape (%) pattern - negative value
java.lang.IllegalArgumentException: URLDecoder: Illegal hex characters in escape (%) pattern - negative value
	at sttp.model.internal.Rfc3986$.decode(Rfc3986.scala:76)
	at sttp.model.UriInterpolator$UriBuilder$.sttp$model$UriInterpolator$UriBuilder$$decode(UriInterpolator.scala:665)
	at sttp.model.UriInterpolator$UriBuilder$.$anonfun$tokensToStringSeq$5(UriInterpolator.scala:620)
	at scala.collection.StrictOptimizedIterableOps.flatMap(StrictOptimizedIterableOps.scala:118)
	at scala.collection.StrictOptimizedIterableOps.flatMap$(StrictOptimizedIterableOps.scala:105)
	at scala.collection.immutable.Vector.flatMap(Vector.scala:113)
	at sttp.model.UriInterpolator$UriBuilder$.doToSeq$1(UriInterpolator.scala:618)
	at sttp.model.UriInterpolator$UriBuilder$.sttp$model$UriInterpolator$UriBuilder$$tokensToStringSeq(UriInterpolator.scala:632)
	at sttp.model.UriInterpolator$UriBuilder$Path$.pathFromTokens(UriInterpolator.scala:479)
	at sttp.model.UriInterpolator$UriBuilder$Path$.$anonfun$fromTokens$1(UriInterpolator.scala:475)
	at sttp.model.UriInterpolator$UriBuilder$.sttp$model$UriInterpolator$UriBuilder$$fromStartingToken(UriInterpolator.scala:565)
	at sttp.model.UriInterpolator$UriBuilder$Path$.fromTokens(UriInterpolator.scala:475)
	at sttp.model.UriInterpolator$.$anonfun$interpolate$1(UriInterpolator.scala:57)
	at scala.collection.LinearSeqOps.foldLeft(LinearSeq.scala:183)
	at scala.collection.LinearSeqOps.foldLeft$(LinearSeq.scala:179)
	at scala.collection.immutable.List.foldLeft(List.scala:79)
	at sttp.model.UriInterpolator$.interpolate(UriInterpolator.scala:56)
	at sttp.model.UriInterpolator$UriContext.uri(UriInterpolator.scala:36)

Originally posted by @zikolach in #257 (comment)