Inconsistent dialect check (sqlserver)

[elgohr]

When using goose.SetDialect multiple values are allowed for dialect.Sqlserver

goose/dialect.go

Line 42 in 6a70e74

case "mssql", "azuresql", "sqlserver":

When using sqlserver as a dialect in goose.NewProvider it results in unknown dialect: "sqlserver" as

goose/database/dialect.go

Line 50 in 6a70e74

querier, ok := lookup[dialect]

doesn't respect sqlserver.

[mfridman]

Happy to merge a fix if anyone is up for submitting a PR.

[globalchubby]

@mfridman: Could you PTAL at #1021 when you have time?