Improve uri.parseQuery to never raise an error

In case of malformed query string where there is `=` on the value, handle
this character as part of the value instead of throwing an error.

The following query string should no longer crash a program:

    key=value&key2=x=1

It will be interpreted as [("key", "value"), ("key2", "x=1")]

This is correct according to latest WhatWG's HTML5 specification
recarding the urlencoded parser:
https://url.spec.whatwg.org/#concept-urlencoded-parser

Older behavior can be restored using the -d:nimLegacyParseQueryStrict
flag.

lib/pure/uri.nim

@@ -165,18 +165,21 @@ iterator decodeQuery*(data: string): tuple[key, value: TaintedString] =
   ## data consists of.
   runnableExamples:
     import std/sugar
-    let s = collect(newSeq):
-      for k, v in decodeQuery("foo=1&bar=2"): (k, v)
-    doAssert s == @[("foo", "1"), ("bar", "2")]
+    let s = collect:
+      for k, v in decodeQuery("foo=1&bar=2=3"): (k, v)
+    doAssert s == @[("foo", "1"), ("bar", "2=3")]
 
-  proc parseData(data: string, i: int, field: var string): int =
+  proc parseData(data: string, i: int, field: var string, sep: char): int =
     result = i
     while result < data.len:
-      case data[result]
+      let c = data[result]
+      case c
       of '%': add(field, decodePercent(data, result))
       of '+': add(field, ' ')
-      of '=', '&': break
-      else: add(field, data[result])
+      of '&': break
+      else:
+        if c == sep: break
+        else: add(field, data[result])
       inc(result)
 
   var i = 0
@@ -185,16 +188,20 @@ iterator decodeQuery*(data: string): tuple[key, value: TaintedString] =
   # decode everything in one pass:
   while i < data.len:
     setLen(name, 0) # reuse memory
-    i = parseData(data, i, name)
+    i = parseData(data, i, name, '=')
     setLen(value, 0) # reuse memory
     if i < data.len and data[i] == '=':
       inc(i) # skip '='
-      i = parseData(data, i, value)
+      when defined(nimLegacyParseQueryStrict):
+        i = parseData(data, i, value, '=')
+      else:
+        i = parseData(data, i, value, '&')
     yield (name.TaintedString, value.TaintedString)
     if i < data.len:
-      if data[i] == '&': inc(i)
-      else:
-        uriParseError("'&' expected at index '$#' for '$#'" % [$i, data])
+      when defined(nimLegacyParseQueryStrict):
+        if data[i] != '&':
+          uriParseError("'&' expected at index '$#' for '$#'" % [$i, data])
+      inc(i)
 
 func parseAuthority(authority: string, result: var Uri) =
   var i = 0