Fix node's parameter names assignment problem

[phelrine]

This commit fixed the issues(#1052, #1139) on which the parameters cannot be obtained on some paths defined as having the same prefix but with different parameter names.
This issue is caused because the node.pnames of shorter paths is overwritten by the node of longer paths inserted.
With this change, I managed to apply the changes to the pnames of a specific node only when the appending node corresponds to the end of URL's path.

I added the following test cases (#1052 and #1139). We have confirmed that the behavior is as expected.
However, this commit does not fix the case (#1139) found below because it needs to store ppath and pnames for each HTTP method like a methodHandler.

func main() {
    e := echo.New()
    e.GET("/order/:shopid", func(c echo.Context) error {
        return c.NoContent(http.StatusOK)
    })
    e.DELETE("/order/:ref", func(c echo.Context) error {
        log.Printf("params %v", c.ParamNames())
        foo := c.Param("ref")
        return c.String(http.StatusOK, foo)
    })
    e.Logger.Fatal(e.Start("127.0.0.1:8080"))
}

[codecov]

Codecov Report

Merging #1201 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1201   +/-   ##
=======================================
  Coverage   81.51%   81.51%           
=======================================
  Files          25       25           
  Lines        1920     1920           
=======================================
  Hits         1565     1565           
  Misses        248      248           
  Partials      107      107

Impacted Files	Coverage Δ
router.go	93.3% <100%> (ø)	⬆️

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[vishr]

@phelrine thanks for your contribution