`roots` sometimes cannot find the correct root(s)

MWE:
```julia
using Dierckx
x = [
-7.800000000000001,
-7.7,
-7.6000000000000005,
-7.5,
-7.4,
-7.300000000000001,
];

y = [
-0.02749549159389436,
-0.01838389014507258,
-0.00921755334192842,
1.1102230246251565e-16,
0.009265146320801443,
0.018574158954026365,
];

spline = Spline1D(x,y)
roots(spline) #! No root

spline = Spline1D(x,y,s=1.0)
roots(spline) # correct

y2 = [
-0.02749549159389436,
-0.01838389014507258,
-0.00921755334192842,
0.0,
0.009265146320801443,
0.018574158954026365,
];

spline = Spline1D(x,y2)
roots(spline) # correct

y3 = [
-0.02749549159389436,
-0.01838389014507258,
-0.00921755334192842,
1.1102230246251565e-14,
0.009265146320801443,
0.018574158954026365,
];

spline = Spline1D(x,y3)
roots(spline) #! No root

y4 = [
-0.02749549159389436,
-0.01838389014507258,
-0.00921755334192842,
1.1102230246251565e-13,
0.009265146320801443,
0.018574158954026365,
];

spline = Spline1D(x,y4)
roots(spline) # correct
```

Output:
```julia
julia> using Dierckx

julia> x = [
-7.800000000000001,
-7.7,
-7.6000000000000005,
-7.5,
-7.4,
-7.300000000000001,
];

julia>

julia> y = [
-0.02749549159389436,
-0.01838389014507258,
-0.00921755334192842,
1.1102230246251565e-16,
0.009265146320801443,
0.018574158954026365,
];

julia>

julia> spline = Spline1D(x,y)
Spline1D(knots=[-7.800000000000001, -7.6000000000000005, -7.5, -7.300000000000001], k=3, extrapolation="nearest", residual=0.0)

julia> roots(spline) #! No root
Float64[]

julia>

julia> spline = Spline1D(x,y,s=1.0)
Spline1D(knots=[-7.800000000000001, -7.300000000000001], k=3, extrapolation="nearest", residual=1.5429392612813467e-15)

julia> y4 = [
-0.02749549159389436,
-0.01838389014507258,
-0.00921755334192842,
1.1102230246251565e-13,
0.009265146320801443,
0.018574158954026365,
];

julia>

julia> spline = Spline1D(x,y4)
Spline1D(knots=[-7.800000000000001, -7.6000000000000005, -7.5, -7.300000000000001], k=3, extrapolation="nearest", residual=0.0)

julia> roots(spline) # correct
1-element Vector{Float64}:
-7.500000000000101
```

Environment:
```julia
julia> versioninfo()
Julia Version 1.7.2
Commit bf53498635 (2022-02-06 15:21 UTC)
Platform Info:
OS: Windows (x86_64-w64-mingw32)
CPU: Intel(R) Xeon(R) W-10885M CPU @ 2.40GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-12.0.1 (ORCJIT, skylake)
Environment:
JULIA_DEPOT_PATH = E:\.julia
JULIA_NUM_THREADS = 8
JULIA_EDITOR = code

(@v1.7) pkg> st Dierckx
Status `E:\.julia\environments\v1.7\Project.toml`
[39dd38d3] Dierckx v0.5.2
```

Start by running the reported MWE with Julia 1.7.2 and Dierckx v0.5.2, comparing roots(spline) across y, y2, y3, and y4. Trace the roots behavior for the Spline1D instances and verify that the expected root near -7.5 is found consistently without relying on smoothing.

Written by the indexing model from the issue text.