Make sure that blueprints with no slash is maintained when applied

[ahopkins]

Resolves #2080
Replaces #2079

[codecov]

Codecov Report

Merging #2085 (4cdf229) into master (4998fd5) will not change coverage.
The diff coverage is 100.000%.

@@            Coverage Diff            @@
##            master     #2085   +/-   ##
=========================================
  Coverage   92.253%   92.253%           
=========================================
  Files           38        38           
  Lines         3485      3485           
  Branches       583       583           
=========================================
  Hits          3215      3215           
  Misses         183       183           
  Partials        87        87           

Impacted Files	Coverage Δ
sanic/blueprints.py	100.000% <100.000%> (ø)
sanic/mixins/routes.py	94.898% <100.000%> (ø)

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[ashleysommer]

Not sure if this covers the condition when BP's (url_prefix == "" or url_prefix is None) and route == "".

[ahopkins]

bp = Blueprint("test")

@bp.get("")
def bar(req):
    return text("bar")

$ curl localhost:8888/ localhost:8888
bar
bar