Drop require checks for unknown schemes

[omus]

julia> using URIs

julia> uri = URI("postgresql://localhost:5432/postgres")
URI("postgresql://localhost:5432/postgres")

julia> URI(uri; userinfo="user:password")
ERROR: ArgumentError: backtrace() requires scheme in uses_authority || isempty(host)
Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/URIs/o9DQG/src/debug.jl:52 [inlined]
 [2] URI(uri::URI; scheme::SubString{String}, userinfo::String, host::SubString{String}, port::SubString{String}, path::SubString{String}, query::SubString{String}, fragment::SubString{String})
   @ URIs ~/.julia/packages/URIs/o9DQG/src/URIs.jl:70
 [3] top-level scope
   @ REPL[4]:1