Better error message when `path` argument does not start with a forward slash

[CameronBieganek]

Example:

julia> HTTP.URI(; scheme="https", host="julialang.org", path="foo/bar")
ERROR: ArgumentError: merge(::HTTP.URIs.URI; scheme::String, userinfo::SubString{String}, host::String, port::SubString{String}, path::String, query::SubString{String}, fragment::SubString{String}) requires !(scheme in ["http", "https"]) || (isempty(path) || path[1] == '/')
Stacktrace:
 [1] macro expansion at /Users/bieganek/.julia/packages/HTTP/GkPBm/src/debug.jl:52 [inlined]
 [2] merge(::HTTP.URIs.URI; scheme::String, userinfo::SubString{String}, host::String, port::SubString{String}, path::String, query::SubString{String}, fragment::SubString{String}) at /Users/bieganek/.julia/packages/HTTP/GkPBm/src/URIs.jl:81
 [3] #URI#3 at /Users/bieganek/.julia/packages/HTTP/GkPBm/src/URIs.jl:66 [inlined]
 [4] top-level scope at REPL[31]:1

That's not a terribly helpful error message. If you look really closely you can see the (isempty(path) || path[1] == '/') part at the end, but it's easy to miss.

In addition to a better error message, it would be a good idea to explicitly mention in the docstring that path must start with a forward slash.

[quinnj]

PR #73 provides component-specific errors for invalid path and authority combinations. It will close this issue when merged.

[investigated by codex; reviewed by quinnj]