Go 1.15: conversion from int64 to string yields a string of one rune

[eclipseo]

Go 1.15 rc 1 on Fedora Rawhide:

Testing    in: /builddir/build/BUILD/macaron-1.3.9/_build/src
         PATH: /builddir/build/BUILD/macaron-1.3.9/_build/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin
       GOPATH: /builddir/build/BUILD/macaron-1.3.9/_build:/usr/share/gocode
  GO111MODULE: off
      command: go test -buildmode pie -compiler gc -ldflags "-X gopkg.in/macaron.v1/version=1.3.9 -extldflags '-Wl,-z,relro -Wl,--as-needed  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld '"
      testing: gopkg.in/macaron.v1
gopkg.in/macaron.v1
# gopkg.in/macaron.v1
./static.go:186:29: conversion from int64 to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./static_test.go:200:23: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./static_test.go:214:23: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL	gopkg.in/macaron.v1 [build failed]

See golang/go#32479